skills(running-tend): add weekly nushell pin bump (#2443)

## Summary

Adds a `Weekly Maintenance: Nushell Pin` section to `running-tend` so
the weekly job picks up upstream nushell releases. PR #2440 pinned
`hustcer/setup-nu@v3` to a concrete version after the `version: '*'`
path went flaky under GitHub API rate limiting; without an ongoing bump
task the pin will drift.

The new section sits next to the existing weekly entries
(MSRV/Toolchain, Statusline Cache-Check) and points at all three
`setup-nu` invocations in `.github/workflows/ci.yaml`.

Requested in
#2440 (comment).

## Test plan

- [ ] Skill rendering: section appears under the existing "Weekly
Maintenance:" headings, no formatting regressions.

---------

Co-authored-by: worktrunk-bot <254187624+worktrunk-bot@users.noreply.github.com>
Co-authored-by: worktrunk-bot <worktrunk-bot@users.noreply.github.com>
Co-authored-by: Maximilian Roos <5635139+max-sixty@users.noreply.github.com>

Author: worktrunk-bot

.claude/skills/running-tend/SKILL.md

@@ -181,6 +181,12 @@ Commit `flake.lock` alongside the other toolchain changes. After bumping, run
 the full test suite (`cargo run -- hook pre-merge --yes`) and verify
 `cargo msrv verify` passes.
 
+## Weekly Maintenance: CI Pin Bumps
+
+Bump pinned third-party versions in `.github/workflows/ci.yaml` to track upstream:
+
+- **`hustcer/setup-nu@v3`** — set `version:` to the latest from `gh api repos/nushell/nushell/releases/latest --jq '.tag_name'` and update all three call sites (`test`, `benchmarks`, `code-coverage`).
+
 ## Weekly Maintenance: Statusline Cache-Check
 
 Detect new in-process cache-miss duplicates introduced by recent changes by